The specific aims of this project are to (1) determine if CFS patient muscle fatigue can be reliably quantified by an isokinetic apparatus, (2) determine CFS patient exercise capacity with bicycle ergometry, (3) determine if CFS patient fatigue measurements differ from healthy or ill control persons, (4) determine if changes in fatigue as measured by isokinetic apparatus correlates with exercise measured by bicycle ergometry, and (5) determine if perceived fatigue correlates with measured fatigue as quantitated by either instrument. The working hypothesis is that fatigue in CFS patients is a biological consequence of their illness and not an alteration in perception. To test this hypothesis, the muscles most frequently involved in patient complaints (leg muscle) will be evaluated by isokinetic and bicycle ergometry exercises. Patient perception of fatigue will be simultaneously determined. Identification of a simple reliable test for determination of muscle fatigability is mandatory in any illness in which fatigue is the central component both for diagnostic and evaluation of therapy purposes.